1. Field of the Invention
The present invention relates to an apparatus for polishing a wafer and more particularly to an apparatus for polishing the chamfered portion along the periphery of the wafer.
2. Description of the Prior Art
Conventionally a silicon single crystal wafer, a compound semiconductor wafer or the like (hereinafter referred to as a wafer) is provided with a chamfered portion along the periphery of the wafer by grinding off the periphery portion.
A wafer with the thus ground off chamfer is not free from generation of fine particles thereon in being handled physically or even in mask alignment in a semiconductor device fabricaton process, though it may prevent the chamfer of its own from cracking and chipping off. Therefore, the wafer with the grinding finish on the chamfer is subject to reduce a yield of devices in semiconductor device fabrication as well as deteriorate the reliability in performance. Under such circumstances, it has been a traditional way to have the chamfer of grinding finish polished.
In the polishing, the ground chamfer is generally pressed to a polishing buff which is rotating about its axis and at that same time the polishing spot is fed with a polishing slurry (fine abrasive grains) which is composed of an alkaline solution with colloidal silica dispersed therein.
When the polishing slurry is fed to the polishing spot on the chamfer of a wafer, the slurry is sprayed on other parts than the polishing spot (for instance a front face or back face of the wafer) and thereby the corrosive action of the alkaline substance included in the slurry gives birth to surficial flaws in the area affected by the alkaline substance. These flaws are not able to be removed away in a cleaning step which is applied to the wafer to get rid of the slurry residue. The flaws on the front face of the wafer is not problematic since a mirror-polishing is applied to the front face and thereby the flaws are removed together with a stock polished off, but to this contrary, those on the back face are left unaffected as it was all the way through the last stage of a wafer fabrication on the product and becomes a new particle source in the following stages and thereby the yield in a semiconductor device fabrication is adversely affected as well as the characteristics of devices thus produced are degraded.
In view of this situation, an apparatus for polishing the periphery portion of a wafer is recently contrived which uses an abrasive tape, which means a tape supporting fixed abrasive grains thereon instead of an apparatus in which a wafer is polished with the help of a polishing slurry. In the former apparatus, the problem of flaws on the back face of a wafer does not occur due to lack of a polishing slurry including an alkaline substance. However, this apparatus does not replace loss of the abrasive grains on an abrasive tape with new ones during operation at a working spot, which differs from the free-abrasive-grain polishing above mentioned in this point of argument, and therefore the loss of the fixed abrasive grains and loading of the abrasive layer take place faster, when the same and one abrasive tape is repeatedly used. Consequently it is indispensable in polishing a chamfer by an abrasive tape that a fresh face of the tape should be always fed to a polishing spot on the chamfer so that the polishing may be effectively executed.
In an apparatus for polishing the chamfer of a wafer, which uses a tape holding fixed abrasive grains, the following contrivance has been made that a feed reel feeds a fresh face of the tape to the polishing spot in succession all the time and thereby the polishing does not fail to be effected by a fresh face as well as a already used face continues to be pulled away and the tape is then wound a take-up reel.
An polishing apparatus of this type is schematically shown in FIG. 4. The polishing apparatus 10 includes a wafer holder 11, a feed reel 12 to feed a tape 14 and a take-up reel 13 to wind the used portion of the tape. With the apparatus 10, the tape 14 is fed by the feed reel 12, a fresh face of the tape 14 is continuously brought to contact the polishing spot and a used face thereof is in succession pulled away to be wound by the take-up reel 13, while the tape 14 is tried to use the full width. On the other hand the wafer is kept on rotating as it is held by wafer holder 11 during polishing, so that the rotation may give rise to a relative velocity between the chamfer of the wafer and the tape.
However a polishing apparatus of this type have had the following problems to be solved.
In polishing by an abrasive tape, important conditions are the feed velocity of the tape and the relative velocity between the tape and the working spot on the chamfer under polishing for effective polishing.
The above mentioned polishing apparatus is adapted to freely adjust the velocity of the tape at an operator's option and thereby the fresh face of the abrasive tape is fed at a variable velocity to the working spot. However, specially in the step of processing the orientation flat portion of the wafer, there remains an unsolved problem that a velocity of the tape relative to the working spot is not able to reach a enough value to polish the chamfer since the relative velocity is dependent on not the rotational motion of the wafer but the motion of the tape in side-way oscillation. What's more, in the step of processing the round peripheral portion, there are such problems as the rotating wafer is subject to vibrate due to frequent decentralized vacuum-chucking on a stage or the full width of a tape is unable to utilize in order to make the finish all over a chamfer uniform and good in quality.